OCTOBER 19-20, 2012 - YMCA University of Science & Technology

Proceedings of the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
work very efficiently within their domain. However, the limitation on the size of the virtual model and its
complexity limits the scope of these systems. A time lag sets in when complex, highly deformable and large sized
virtual models are encountered and real-time interaction may not be possible.There are more research groups [24-
26] using virtual reality for concept design. The virtual concept design tools are in high demand in industry.
1.2. VR Based Design Process
The virtual reality based design process is based on the interaction of the user with the virtual model through a
haptic device. There are three distinct processes that need to be performed to carry out the interaction. To
visualize the virtual model and the tool, a ‘haptic rendering system’ is used. The ‘collision detection system’
calculates the relative position of the model and the tool at any instance in time. As soon as the haptic tool starts
interacting with the tool, this information about the position of contact and the extent of penetration is provided
by the collision detection system. Based the information provided by the collision detection algorithm, the
‘physics-based system’ determines the deformation of the model(s) and the resultant reactive forces to be fed
back to the user. Figure 2 shows the schematic representation of a general virtual reality-based design process.
Figure 2. Schematic Representation Of A Vr-Based Design Process.
1.3. Collision Detection
Collision detection is used in virtual assembly, simulation-based concept design, evaluation of design, motion
planning, medical training, virtual reality based games and animation. Collision detection is computationally
intensive and is considered a bottleneck in these applications. A collision detection system automatically reports a
geometric contact between the haptic tool and a virtual object. It calculates when a contact is about to occur or
has actually occurred. There are many collision detection algorithms available in literature. Jimenez [27] surveyed
various collision detection algorithms. It focused on how the model representation leads to different collision
detection algorithms. On the other hand, Lin and Gottschalk [28] presented a survey on the state of the art in
collision detection between models represented by smooth surfaces. Pungotra et al [29] proposed an algorithm
that uses B-spline based models and rigid as well as deformable tool. This algorithm uses best of parametric
representation of surface and efficiency of triangle-triangle intersection test.
1.4.Physics Based Deformation Modelling
The deformation of the model can be simulated by a geometric- or physics-based system. Geometric techniques,
though efficient, do not yield accurate results which may be required for the evaluation of the model. Physicsbased
techniques, on the other hand, yield accurate results. However, the computational cost is large for such
techniques. A mass spring damper system, consisting of a set of particles (nodes) connected through a network of
springs and dampers can provide reasonable accuracy and speed for real time interaction. Pungotra et al [30]
proposed the representation of B-spline surfaces in terms of blending matrices. This facilitated integration of
collision detection with the mass spring system. The virtual object is modeled as a collection of point masses
connected by springs and dampers in a lattice structure. In general, the spring forces are assumed to be linear.
However, nonlinear springs can also be used to model objects which exhibit inelastic behaviour. Such a system
contains a mass ρ, a spring with spring constant K that serves to restore the mass to a neutral position, and a
damping element which opposes the motion of the vibratory response with a force proportional to the velocity of
the system. The constant of proportionality, also known as damping constant, is denoted by D. Different
combinations of linear springs and damper can be used to model deformable objects. Voigt model is the most
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